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Award Ceremony Speech

The Nobel Prize in Physics to Professor Tsung Dao Lee and
Professor Chen Ning Yang for this year is concerned with some of
the fundamental physical principles, the so-called parity laws -
in the first place the symmetry of Nature with respect to right
and left - in their application to elementary particles and their
reactions.

When during this century the old dream about atoms came true, it
soon became clear not only that reality is by far richer than was
the dream but also considerably different from it. The atoms that
were found and which one learnt to count and to measure did by no
means correspond to the ideal of indivisibility and
unchangeability of the old atomists. But instead, there appeared
a hitherto unknown, strangely unvisualizable feature of
indivisibility in all atomic processes and therefore also behind
all usual physical events, without which everything in the world
would be in flux. The new edifice of laws, which was the
consequence of these discoveries - it is called quantum theory -
contains the laws of earlier physics as a correct but greatly
simplified limiting case. They have a similar relationship to the
laws of atomic physics as an airphoto to a near-picture of the
same landscape.

The lesson learnt from quantum theory made that the literally
unchangeable atoms of the old philosophers were hardly seriously
sought in those particles-electrons, protons, and neutrons - from
which atoms are made. As the name elementary particles under
which they are summarized would seem to indicate, there was
nevertheless a certain inclination towards this direction. But
already the ordinary elementary particles are by no means
unchangeable, which is still more strongly the case with the lot
of new, similar particles discovered during later years, the
transformations of which now stand at the centre of interest of
atomic physicists. In our attempts to find a theory which
comprehends all the new facts concerning old and new elementary
particles, certain wittingly unreal, symbolic particles appear in
our equations, which with a little good will could be regarded as
the eternally immutable atoms of philosophers. Of the real
elementary particles we could then, following Lao-tse, the old
Chinese thinker, say: "The elementary particles, which could be
defined, are not the eternal elementary particles". Lao-tse did
not talk of elementary particles, of course, but of Tao, the
deepest principle of life. And physics is certainly considerably
simpler than human life. And there we have powerful auxiliaries
in the art of experimentation and mathematics.

As to mathematics and elementary particles it has, in the first
place, led to two theories, each of which has been developed by a
Nobel Prize winner in Physics, Dirac and Fermi. Hereby the former
theory is the outermost wing of the edifice of quantum mechanics,
while the latter may be regarded as the first, still unfinished
room in the new edifice of elementary particle laws. But they are
both concerned with electrons and thus border on one
another.

But what has the question of right and left to do with elementary
particle physics? Well, in the first place only in a negative
way, in that it was assumed almost tacitly, that elementary
particle reactions are symmetric with respect to right and left.
This assumption was to play an important part in the elaboration
of Fermi's theory. That this assumption was made was very
natural, not least in view of the mentioned theory of Dirac,
according to which it looked as if the electrons, the best known
elementary particles, possessed no feature which would permit a
distinction between right and left. In fact, most of us were
inclined to regard the symmetry of elementary particles with
respect to right and left as a necessary consequence of the
general principle of right-left symmetry of Nature. Thanks to Lee
and Yang and the experimental discoveries inspired by them we now
know that this was a mistake.

The starting-point of Lee and Yang in their revision of the whole
question of right-left symmetry in elementary particle reactions
were certain strange observations concerning a kind of new
particles called K mesons, which looked as if they were in
contrast with the assumption mentioned. Even if these
observations puzzled greatly many physicists, it was only Lee and
Yang who seriously took the consequences of them, in that they
asked themselves what kind of experimental support there was for
the assumption that all elementary particle processes are
symmetric with respect to right and left. The result of their
investigation was unexpected, namely that the validity of the
symmetry assumption even in the best known processes had no
experimental support whatsoever, the reason being that all
experiments had been so arranged as to give the same result
whether the assumption was valid or not. As if one had thought
that Olav Tryggveson had his heart in the middle of the body
because he was equally skilled with the left as with the right
hand. Lee and Yang did not confine themselves to this negative
statement but devised a number of experiments which would make it
possible to test the right-left symmetry in different elementary
particle transformations, and proposed them to their experimental
colleagues. The first of these experiments was carried out by the
Chinese physicist, Mrs. C.S. Wu and her collaborators. Very
schematically it consisted in the following. Atomic nuclei of a
radioactive isotope of the metal cobalt were exposed at very low
temperature to a magnetic field - they are themselves small
magnets - whereby they became directed just like compass needles.
The distribution as to direction of the electrons due to
radioactivity was then investigated.

Let us assume that the magnetic field is created by means of a
coil placed like a spool of thread on a table, and that the
electric current is flowing counterclockwise in the wire. Then
the north poles of the cobalt nuclei will be directed upwards.
The experiment, now, gave the result that the electrons from the
radioactive process with this arrangement were preferentially
thrown downwards towards the floor. From this it follows
unambiguously that the process lacks that right-left symmetry,
which one had earlier assumed. Thus, by means of this experiment
it could be explained to a person, who did not know it - let us
say an inhabitant of a distant stellar system - what we mean by
right and left. In fact, it would be sufficient to ask him to
arrange the experiment so as to make the preferential direction
of the electrons point downwards. The current will then have the
same direction as that in which he has to turn at the command
"left face".

However - and this is a thing of the utmost importance for the
incorporation of the new discoveries into our edifice of physical
laws - the person on the distant planet will be able to follow
our prescriptions only if he knows what we mean by the direction
of an electric current. And to know this he must know that our
atoms and his are made up of the same elementary particles. We
know, however, that not only are there double sets of electrons -
positive and negative - but that the same holds for protons and
neutrons, the building stones of atomic nuclei. It is therefore
possible that his atoms contrary to ours would consist of
positive electrons and negative nuclei. If they did, he would
judge the direction of the current opposite to what we would do,
with the result that he would call right left and left right. In
stating this we have tacitly made an assumption which is not
quite confirmed as yet but which, as far as the experiments go,
seems probable, namely that the results of all experiments
performed with the opposite kind of elementary particles would be
just such as to reestablish the right-left symmetry. With other
words, one should be able to regard the antiparticles not only as
the electric opposites of the particles but also as their mirror
images.

Professor Lee and Professor Yang. In the
very incomplete sketch of your new work I have just made in
Swedish, time has not allowed me to mention the many other
beautiful contributions to theoretical physics made by each one
of you, nor could I at all do justice to the enthusiasm your new
achievement has aroused among physicists. Through your consistent
and unprejudiced thinking you have been able to break a most
puzzling deadlock in the field of elementary particle physics
where now experimental and theoretical work is pouring forth as
the result of your brilliant achievement.

It is with great satisfaction, therefore, that The Royal Swedish
Academy of Sciences has decided to award you this year's Nobel
Prize for Physics for your fundamental contributions to this
branch of science.

On behalf of the Academy I wish to extend to you our heartiest
congratulations and now ask you to receive from the hands of His
Majesty the King the Nobel Prize for Physics for the year
1957.